The present invention generally relates to an automatic implantable atrial defibrillator for delivering cardioverting electrical energy to the atria of a human heart when the atria are in need of cardioversion. The present invention is more particularly directed to such an atrial defibrillator which detects for ischemia of the heart before therapy intervention. If ischemia is detected, a normal therapy intervention process is altered, as by withholding application of cardioverting electrical energy until the ischemia is no longer detected.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience reduced cardiac output and an increased irregular heart rate resulting in palpitations of the heart and even dizziness or loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide relief to patients suffering from occurrences of atrial fibrillation. One such implantable atrial defibrillator is described in U.S. Pat. No. 5,282,837, which issued on Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness, for ATRIAL DEFIBRILLATOR AND METHOD, and which is assigned to the assignee of the present invention and incorporated herein by reference. The defibrillator there described includes an atrial fibrillation detector for detecting atrial fibrillation, and a cardiovertor for cardioverting the heart.
Patients who suffer from atrial fibrillation may also suffer from myocardial ischemia. Myocardial ischemia is a condition wherein there is insufficient blood supply to the myocardium (muscles of the heart) to meet the demand of the myocardium for blood. The ultimate result of persistent ischemia is a necrosis or death of a portion of cardiac muscle tissue, known as a myocardial infarct. Such an occurrence is commonly known as a heart attack.
Insufficient blood supply to the myocardium is generally due to an obstruction or thrombus in an artery which supplies blood to the myocardium. Another cause can be atrial fibrillation, wherein the increased heart rate associated with atrial fibrillation increases the work, and hence the blood demand of the myocardium, while the atrial fibrillation at the same time reduces the blood supply.
Myocardial ischemia is usually detected by submitting the patient to a stress test which increases the cardiac demand of the myocardium. During and after the stress test, twelve lead ECG monitoring is utilized to determine if the heart is receiving an adequate supply of blood.
Ischemia, and the myocardial infarcts that it causes, are known to render the heart unstable and more susceptible or vulnerable to various forms of life threatening arrhythmias. As a result, the present invention contemplates avoiding disturbing the heart during those times in which the heart is experiencing an episode of ischemia. More particularly, and in accordance with a preferred embodiment of the present invention, an implantable atrial defibrillator provides such avoidance by withholding application of cardioverting energy to the atria, even though the atria may require cardioversion, until after the ischemic episode has terminated. In accomplishing this end, before the cardioverting energy is applied, an ischemia detector performs an analysis for possible ischemia. The cardioverting energy is not applied until the ischemia detector fails to detect ischemia of the heart. As a result, the atrial defibrillator not only provides cardioversion of the atria when required, but further provides detection of ischemia when such a condition would otherwise have been permitted to persist unnoticed.